The word originated in Hackney, London. Specifically Kingsland Secondary School (now sadly gone).
The word is a direct product of one Nang Phan, an ex-student of KS. It came about through boys in years above her chiding "ahh, Nang you're nang". It caught on like wildfire from there.

Nitrous Oxide (laughing gas - used as anaesthetic in dentistry) filled little gas cylinders normally used in a
whipped cream syphon ('Nang machine') in cafes etc, but inhaled slowly directly into the mouth or via a filled balloon. Gives a detachment high for around one minute. Term nang thought to come from Western Australia and from the sound distortion you have while affected. It's legal to buy in all states in Australia. Usually accompanies other drugs such as ecstacy pills and acid. Purchased in box of ten from delis, servos and supermarkets.